Generally, a crystal unit, such as a piezoelectric crystal unit, is provided in a package containing a piezoelectric element. Typically, external electrodes disposed on an outer surface of the package are electrically connected to internal electrodes of the package. The internal electrodes are connected to electrodes of the piezoelectric element housed within the package.
The piezoelectric element typically has two electrodes, which may be connected to two internal electrodes of the package by using an electrically conductive adhesive. In one method of connecting the electrodes using electrically conductive adhesive, an amount of the electrically conductive adhesive is applied to the two internal electrodes of the package using a dispenser, and then the two electrodes of the piezoelectric element are placed on the two internal electrodes of the package via the electrically conductive adhesive. The electrically conductive adhesive is thereafter cured. Thus, the piezoelectric element is housed within the package with the electrically conductive adhesive disposed between the two piezoelectric element electrodes and the two internal electrodes of the package.
In this case, the electrically conductive adhesive between the electrodes may flow out and come into contact with another portion of the package, resulting in a short-circuit or changing the oscillating frequency of the crystal unit. A conventional technology aims to overcome the above problem by providing a concave portion in the package for containing the electrically conductive adhesive, while a through-hole is formed in the bottom of the concave portion. The technology allows excess electrically conductive adhesive to flow out of the concave portion via the through-hole (see JP-A-8-316771, for example).
It has also been proposed to provide a concave portion or a through-hole between a portion that is bonded by the electrically conductive adhesive and a portion that is not bonded, so that excess electrically conductive adhesive can be collected by the concave portion or via the through-hole (see JP-A-2007-288644, for example).
Another proposed solution involves increasing the thickness of a front-end part of an electrically conductive adhesive portion compared to the thickness of an intermediate- or rear-part of the adhesive portion when connecting the lead electrodes of a piezoelectric element to the internal electrodes of the package via the electrically conductive adhesive portion (see JP-A-2004-222006, for example). The internal electrodes of the package may be coated with the electrically conductive adhesive by dropping a liquid electrically conductive adhesive onto the internal electrodes from a nozzle of a dispenser. In this case, the amount of the electrically conductive adhesive dropped and the dropped position may vary between the two internal electrodes. FIG. 1 is a plan view of the inside of a package 2 having two internal electrodes 4-1 and 4-2 on which drops of an electrically conductive adhesive 6 have been dropped from a dispenser. It is seen that, due to variations in the dropped amount and position of the electrically conductive adhesive, the size and position of the drops of the electrically conductive adhesive 6 are different between the internal electrodes 4-1 and 4-2.
As illustrated in FIG. 1, when the position or amount of the drops of the electrically conductive adhesive 6 is different between the internal electrodes 4-1 and 4-2, the oscillating frequency of the piezoelectric element may be destabilized, or the piezoelectric element may fail to oscillate. For example, as the electrically conductive adhesive 6 cures, the stress distribution of the piezoelectric element may become uneven, resulting in a change in its oscillating frequency characteristics. The stress distribution of the piezoelectric element may also be changed by the aging of the electrically conductive adhesive 6, resulting in a change in the oscillating frequency characteristics.